Selective adsorption can separate a desired component from a mixed feed by adsorbing the desired component while letting other components in the mixture flow by. The other components are referred to herein as “undesired components” to differentiate them from the desired component, but the undesired components may be used for other purposes or processes and therefore be desirable in their own right. An adsorption separator may use an adsorbent with a higher affinity for the desired component than for the undesired components in the mixture, so the desired component is adsorbed onto the surface and within the pores, cavities, or other areas of the adsorbent. The adsorbent may adsorb some of the desired component, the undesired components, and other compounds, but the more preferred compounds are adsorbed more readily. Selective adsorption can also proceed by adsorbing undesired components and allowing the desired component to flow through the adsorbent for collection. In this description, the desired component is adsorbed by the adsorbent, but this description is also applicable to embodiments where the undesired components are adsorbed and the desired component flows through the adsorbent.
As the mixture flows over the adsorbent, the desired component is adsorbed so the fluid passing through the adsorbent has a lower concentration of the desired component, and therefore a higher concentration of other components. This adsorption process diminishes after a period of time because the available adsorption sites on the adsorbent are taken up. The undesired components in the mixture may then be drained or displaced from the adsorbent in a purification process. Additional fluid flowing through the adsorption bed pushes the undesired components out in a raffinate stream. A desorbent may then be introduced into the adsorbent bed, where the desorbent flushes the desired component from the adsorbent. The desorbent displaces the desired component from the adsorbent in a desorption process, and the desired component can then be collected with some excess desorbent.
In a simulated moving bed separator, a plurality of adsorption beds filled with adsorbent are fluidly connected together and fixed in position. Some simulated moving bed separators use a plurality of adsorption beds fluidly coupled in a circular manner, so fluids flow through the adsorption beds in a loop. i.e., fluid flows from the first adsorption bed into the second adsorption bed, fluid flows from the second adsorption bed into the third adsorption bed, and so on, and fluid from the last adsorption bed flows into the first adsorption bed. The simulated moving bed separator has various zones separated by the inlet and outlet points. The zones shift through the simulated moving bed separator by changing inlet and outlet locations from one adsorption bed to the next. For example, the mixed feed may be introduced into a feed bed, a raffinate stream may be removed from a raffinate bed, the desorbent may be added to a desorbent bed, and the desired component may be removed from an extract bed in an extract stream. A plurality of conduits fluidly couple the adsorption beds to a distributor, and the distributor shifts or changes the feed bed, the raffinate bed, the desorbent bed, and the extract bed in what is referred to as a valve step. The valve step in a typical simulated moving bed separator changes the feed bed from one adsorbent bed to an adjacent adsorbent bed. The valve step also changes the raffinate bed, the desorbent bed, and the extract bed to directly adjacent adsorbent beds. After a period of time, the distributor changes the feed bed and the other named adsorbent beds again, such that each adsorption bed serves as the feed bed, the raffinate bed, the desorbent bed, and the extract bed in turn during what is referred to as a valve cycle.
Zone 4 is one of the zones described above, where zone 4 includes the adsorbent beds between the raffinate bed and the desorbent bed. Undesired components are removed from the simulated moving bed separator in the raffinate bed, and the desorbent is added to the simulated moving bed separator in the desorbent bed. Zone 1 includes the adsorbent beds between the raffinate bed and a feed bed, so fluid that does not exit the raffinate bed in the raffinate stream flows from zone 1 to zone 4. The circular fluid flow in zone 4, while still in the positive direction, is significantly slowed relative to other zones in order to control the residence time of contaminants that bypass the raffinate stream and cross from zone 1 into zone 4. Residence time in the adsorbent bed immediately below the raffinate bed should be sufficiently high so that the undesired components are not carried out of that adsorbent bed, and residence time is increased by slowing fluid flow. With sufficient residence time, the undesired components remain in the adsorbent bed immediately below the raffinate bed until the valve step, at which time the adsorbent bed immediately below the raffinate bed becomes the raffinate bed, and the undesired components can be washed out with the raffinate stream. If sufficient residence time in zone 4 is not maintained, contaminants will enter zone 3 and exit the simulated moving bed separator in the extract stream. This increases the quantity of undesired component in the extract, resulting in a less complete separation. Manipulating the flow rate in zone 4 typically involves increasing the rate at which the raffinate stream is removed from the raffinate bed (thus lowering the zone 4 flow rate and increasing the residence time in zone 4). To maintain overall balance, extra desorbent is added to the desorbent bedn with an increase in the raffinate stream flow rate. The extra desorbent added to the process is traditionally separated from the desired components and the undesired components by distillation, and more energy is required for the distillation when more desorbent is present.
The simulated moving bed separator simulates countercurrent movement of liquid and solid, where the solid is the adsorbent. In a true moving bed, solid adsorbent moving counter currently to liquid will carry some amount of liquid within the pores of the adsorbent. This “flow” of liquid suspended within the pores of the adsorbent is subtracted from the free liquid flow countercurrent to the adsorbent in order to arrive at a calculated “net” flow of liquid. Since the control equations of a true moving bed are the same as those of a simulated moving bed, those skilled in the art frequently refer to the liquid flow within zone 4 as negative. In other words, within zone 4 of a true moving bed, the flow of liquid suspended within the pores of the moving adsorbent may be greater than the flow of free liquid countercurrent to the adsorbent and thus produce a ‘net’ negative flow. Irrespective of net fluid flow and typical calculations, the actual free liquid flow within zone 4 should be sufficiently small to control resident time within zone 4. The flow of free liquid within zone 4 remains in the positive direction (the same direction as all other zones), even though conventional calculations based on a true moving bed predict a net negative flow in zone 4.
Accordingly, it is desirable to develop methods and apparatus to separate a desired component from a mixture with a lower concentration of undesired components in the extract. In addition, it is desirable to reduce the total amount of energy needed to recover the desired component and the undesired component while reducing the concentration of undesired components in the extract. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.